Can a "Super-Soaker" shatter car window?
Edward G. Hubbard
Hello, I'm working on defending a Juvenille who is
accused of using a "super-soaker" water gun to
shatter a car window. He states that he didn't
squirt the car at all - and notes it is in
a construction zone - that is the scene of the
incident, so we think it likely that a stone,
etc. actually did the damage.
If we can show that it is not possible for the
stream of water from a squirt gun to break the
window, the prosecutor will drop the charges,
and our client, who has no record can get
on with his life.
The circumstances are as follows: the "victim" places
the squirt gun some three to four feet from the window of
her car. It is the driver's side window, which was
fully rolled up. She didn't see our client actually
squirt the gun, but only saw him pointing it and then
"my window shattered into a million pieces." She was
driving probably under 25 mph.
I would greatly appreciate any opinions or expertise
regarding this.....When I consider the pressure that
a high pressure nozzle in a car wash generates I
just don't think its possible. Also, they have
only sold 4 million of these guns, and there
is not a single lawsuit out there indicating
similar damage.
If you have any further questions regarding the
situation, please let me know.
Thank you,
Ed.
Flint Smith
..
>If we can show that it is not possible for the
>stream of water from a squirt gun to break the
>window, the prosecutor will drop the charges,
>and our client, who has no record can get
>on with his life.
>The circumstances are as follows: the "victim" places
>the squirt gun some three to four feet from the window of
>her car. It is the driver's side window, which was
>fully r...
How's this? The "vandal" is wandering through a
construction site looking for trouble. He finds that
a pipe cap fits snugly over the end of the supersoaker
and that it flies nicely when propelled by the stream of
water. Being a criminal-in-training, it comes naturally
for him to point his gun at passing motorists. Being only
interested in guns and destruction, he doesn't know how
fragile safety-glass is when subjected to point-source
impacts, or maybe he doesn't care.
>I would greatly appreciate any opinions or expertise
>regarding this.....
I think my opinion is clear: Let him pay.
Flint Smith fl...@uclaue.mbi.ucla.edu
Tim Kelly
>
>
>Hello, I'm working on defending a Juvenille who is
>accused of using a "super-soaker" water gun to
>shatter a car window.
Not sure this is pertinent but . . .
Did you ever think that the glass broke due to thermal expansion/contraction
properties of glass. This is usually characterized as coeffient of thermal
expansion (CTE).
If the glass was very hot and the water cold it could have broken. This has
happened to me twice with windshield. (Car sat in driveway and got hot. I
turned the hose on it to wash it.)
On the other hand, windshields are large and can easily develope high CTE
stresses. Intuitively, a side wind is small (less affected by CTE).
TKK
Brian W. Tucker
Flint Smith is a complete idiot. No one mentioned pipe caps.
Besides, asshole, have you ever heard of "innocent until proven
guilty"? Lot's of kids play near construction sites - they are
everywhere and there is 'neat stuff'. There is no evidence from
the original post that the (boy?) is "a criminal in training".
To everyone else, sorry this response is a bit off the subject
of this news group, but the elitist arrogance (and shear
stupidity) of people like this can't go unchallanged.
I think the original question was very interesting, and would
love to hear a scientific answer from someone else out there,
although I know from experience that there is no way a super
soaker could break a car window. (It's water, at (relatively)
low pressure for God's sake - a super soaker isn't even close
to the garden hose that you regularly wash a car with, not to
mention the high pressure hoses at very close range that some
car washes use.) Furthermore, that a super soaker could propel
a pipe cap with such force as to shatter a window, as this
jerk has suggested, is nonsensical in my opinion. Again, low
pressure... Besides, haven't we all expereinced a truck kicking
up a rock on the highway and it hitting your windshield? So
you're driving 65 mph in one direction, the rock flies back at
you with at least some force in the other, and your windshield
*may* recieve a small crack - maybe not even that. So do you
really think a super soaker can shatter a window?
--
Jeff Smith
: I think the original question was very interesting, and would
: love to hear a scientific answer from someone else out there,
: although I know from experience that there is no way a super
: soaker could break a car window. (It's water, at (relatively)
: low pressure for God's sake - a super soaker isn't even close
: to the garden hose that you regularly wash a car with, not to
: mention the high pressure hoses at very close range that some
: car washes use.)
This leads to a good way of figuring out whether or not a super soaker
could break the window. The company that makes them should be able
to give you the maximum pressure of the water in the gun. Assume that
the flow is approximately the same as that of a garden hose with a
hand nozzle attached. Ask a company that makes hoses or nozzles what
the maximum pressure of the water is that comes out of the hose.
--
Ivan Clark
>
>
>Hello, I'm working on defending a Juvenille who is
>accused of using a "super-soaker" water gun to
>squirt the car at all - and notes it is in
>a construction zone - that is the scene of the
>incident, so we think it likely that a stone,
>etc. actually did the damage.
>
>stream of water from a squirt gun to break the
>window, the prosecutor will drop the charges,
>and our client, who has no record can get
>on with his life.
I think that you will have trouble proving that there is
absolutely no way that a high pressure, high volume squirt
gun loaded with an indeterminate material and being fired by someone
standing 3-4 feet from the drivers side window of a moving car
could break the window of that car. Further, while you
have to defend your client, calling the juvenile the victim seems
to imply that the person whose window shattered onto them
while driving down the street is somehow the guilty party.
It is my understanding that, in the Commonwealth of Virginia,
it is a felony to propel a projectile of any sort at the driver
of a moving vehicle. This definitely includes snowballs and might
be taken by extension to include supersoakers since they do not
neccessarily contain water (some kids at Virginia Beach some years
ago apparently thought it would be more fun to use
mouthwash or aftershave) (and even water fired into the eyes of a
driver can cause them to lose control of the vehicle). So, up front,
if your client stood in the street and fired a supersoaker at a
driver in a moving vehicle, you should be glad that I am not on the
jury.
>The circumstances are as follows: the "victim" places
>the squirt gun some three to four feet from the window of
>her car. It is the driver's side window, which was
>fully rolled up. She didn't see our client actually
>squirt the gun, but only saw him pointing it and then
>"my window shattered into a million pieces." She was
>driving probably under 25 mph.
>
>I would greatly appreciate any opinions or expertise
>regarding this.....When I consider the pressure that
>a high pressure nozzle in a car wash generates I
>just don't think its possible. Also, they have
>only sold 4 million of these guns, and there
>is not a single lawsuit out there indicating
>similar damage.
I would wager that lawsuits over similar damage would get
settled very quickly by the supersoaker manufacturer with a
gag stipulation on the settlement. No public suits certainly
does not disprove the problem.
You have received other comments in this thread about thermal
damage and about rocks. If the window shattered due to cold
fluid from your client (or hot fluid on a very cold day, if
this happened a while back), then I would consider your
client a contributing factor in the failure of the window, but
you can probably get a reduced finding (or even not guilty if
the jury likes your client) by pointing out that the window
had to be defective or improperly installed for this to
happen (and put the blame on the car manufacturer or the last
business that replaced that window). The key here is whether
there have been other occurances of similar window breakage
at carwashes. You could check window installers and dealers
for that model (or even the manufacturer) for high frequency of
replacement. You might even be able to convince the real victim
that having their window break at low speed actually helped them
in the long run as compared to being shattered by a sudden rain
storm on the interstate.
If you have all the pieces of the shattered window, an expert on
fracture might be able to tell you if the failure is due to impact
or thermal stress. CWRU has an excellent engineering school. I would
expect at least one such expert there, but this is not my specialty
area and do not have any specific names for you. I would expect this to
be a very time consuming process, however, for your expert witness.
It seems that you have some options:
1) impact shattering
a) can your client show that they did not use the supersoaker to
fire projectiles? i.e., no scratches in or on end of barrel
i) yes, not guilty
ii) no, you have a problem
2) thermal stress shattering
a) water in supersoaker
-need to show no malicious intent and that
you actually did the driver a favor
b) other than water in supersoaker
-you have a serious challenge
>If you have any further questions regarding the
>situation, please let me know.
>
>Thank you,
>
>Ed.
If this is truly a young kid who did not know any better, I wish
you well (and I hope their parents will teach them not to play in the
street before they get run over).
Ivan Clark
David Allsopp
you could not break a car window with one by mechanical force. Remember,
these are designed to be fired by kids at each other's faces without
injury (or there really *would* be trouble in the courts!). The barrels
are too small to fire a solid projectile of any note. The thermal shock
idea is interesting, but firstly you could see how hot the water could be
(the plastic of the gun might soften in near-boiling water), and the
temperature on the day in question must be available, so you could find out
whether there was any possibility of cracking due to this. Since people
regularly clean their cars with hot water on cold days, I doubt there is much
chance of cracking.
David.
-------------------------------------------------------------------------------
The modulus of elasticity of any substance is a column of the same substance,
capable of producing a pressure on its base which is to the weight causing a
certain degree of compression as the length of the substance is to the
diminution of its length. - Thomas Young, 1807.
-------------------------------------------------------------------------------
FolsomMan
...
>If we can show that it is not possible for the
>stream of water from a squirt gun to break the
>window, the prosecutor will drop the charges,
>and our client, who has no record can get
>on with his life.
Caveat: I am not a licensed professional engineer and am just expressing
opinions here. You need to get someone with credentials to back this
stuff up. Opinion only---
You should be able to get the operating pressure of the squirt gun from
the manufacturer. The maximum pressure inside the gun will always be
greater than the maximum pressure achieved at the window surface, since
the dynamic pressure will be the original stagnation pressure minus any
losses from flow friction. It is certainly not enough to break a sound
(non-defective) window by the pressure force exerted. It seems much more
likely that thermal stress would break the window, starting at an existing
defect. Tempered glass is heat-treated in a way that puts the surface
layers in compression and the interior in tension. This usually works
wonderfully well because typical glass failures involve cracks originating
at a free surface and then propagating through the material when there is
a tension force pulling the crack apart: the compressive stress induced
by the thermal treatment usually keeps such cracks closed. The problem
comes when a defect is present (either originally or damage induced) in
the material that is under tension in the core. All of the elastic
potential energy that is stored in the compressed surfaces and the
stretched core is available to make cracks propagate all over the place
the moment that a defect in the core reaches the critical size, or the
stress in the material increases enough to make an existing crack size
critical. Crack growth can occur slowly in glass that is stressed in
tension and exposed to long term environmental moisture, leading to a
sudden and unexpected catastrophic failure with no apparent proximate
cause. A long-standing unnoticed chip from a stone or an internal bubble
in the glass would both be potential candidates to grow into a critical
sized crack.
>The circumstances are as follows: the "victim" places
>the squirt gun some three to four feet from the window of
>her car. It is the driver's side window, which was
>fully rolled up. She didn't see our client actually
>squirt the gun, but only saw him pointing it and then
>"my window shattered into a million pieces." She was
>driving probably under 25 mph.
>I would greatly appreciate any opinions or expertise
>regarding this.....When I consider the pressure that
>a high pressure nozzle in a car wash generates I
>just don't think its possible. Also, they have
>only sold 4 million of these guns, and there
>is not a single lawsuit out there indicating
>similar damage.
If it could break car windows, just think what it could do to skin or
eyes!!
Mark Folsom
Roland Huet
Hubbard) wrote:
> Hello, I'm working on defending a Juvenille who is
> accused of using a "super-soaker" water gun to
> shatter a car window. He ... places
> the squirt gun some three to four feet from the window of
> her car. It is the driver's side window, which was
> fully rolled up. She didn't see our client actually
> squirt the gun, but only saw him pointing it and then
> "my window shattered into a million pieces."
If the facts are as you describe, it seems highly unlikely that a jet of
water from a super-soaker could break safety glass. After all, the
pressure generated at the point of impingement is not very high, it does
not even penetrate the skin.
Now, if the opening of the gun was covered by some hard cap that is then
launched, as has been suggested in another post, the story may be
different. Some tests or analysis, combined with review of applicable
standards, is probably needed to find out whether there is sufficient
energy in the cap to break the glass. If the pieces of glass are still
available, they should be examined for prior defects. Of course,
examination of the gun itself could show if one could attach a cap on the
end.
E-mail back if you want to discuss this further. We have investigated
thousands of accidents and failures of all sorts.
--
Roland Huet
Keith
window would be far beyond the level of impact at which people
would be seriously hurt by such a jet spray.
I would therefore expect that if there are not normal occurances
of human bodily injury due to this, it is unlikely that a car
window would be broken by it.
This would of course be different if it were used to fire a
projectile.
Why not just try an experiment with the same super soaker, and
see if it will break a car window of the same type (specially
purchased for the trial) under similar circumstances. Then
incrementally weaken the window by scribing grooves to simulate
pre-existing damage, and repeat the trial until it shatters, or
it has been demonstrated that it would realistically not ever
shatter under similar conditions?
Todd Voiles
says...
>
Just a sidebar on this discussion-
The front windshield on cars is NOT tempered (or safety) glass.
It consists of 2 layers of fairly normal glass with a plastic sheet inbetween
If it was tempered then two things would happen:
1) whenever a rock or large object chipped the windshield it would shatter
completely.....bad for driving
2) When an accident occured then anyone not wearing a seatbelt would go
splat on the windshield. As a demonstration once we set up a stand with
some tempered glass in it and let people throw softballs at it at a distance
of less than 20 feet...no one could break it......imagine your body impacting
such a glass.
Oh well.....Just my 2 cents as a person with a Ceramic Engineering Degree
Tom Mayer
a car window, it would be powerful enough to damage a child's eye or
break skin. If the window had been previously damaged by a chip or
scratch deeper than about 30 microns (about .0012") it could be possible
that the cooling effect of water evaporating on a warm glass surface
could set up thermal gradian. The stress caused by a thermal
differencial of this type could caused a damaged window to self destruct.
I am sure we have all heard the story of someone who came back to their
car after a day at the beach to find the back window of their car had
exploded. Car glass is tempered. It is heat treated such that the out
side 25-30 microns of glass are under expansive stress while the inside
of the glass is under compressive stress. If one were to push on the
glass the stress on the surface is lowered. If it is hit hard enough to
damage this thin expansive layer the system equilibrium is disrupted and
the glass shatters into thousands of small pieces. Safety glass is
designed to shatter into small pieces so that large chunks of glass are
not thrown at passengers in an accident. A BB, nail, or other hard
object could have been placed in the soaker and shot with enough force to
break the window. Bottom line the window would not have been broken if
the kid did not shot at it. I had an apple hit my front car window at 35
MPH (thrown by a kid) and it sounded like a bomb went off. Nothing was
broken but I almost wrecked the car. What this kid thinks of as a
harmless prank could have caused the driver to panic and cause major
damage and (or) injuries.
Chris Jones
: car after a day at the beach to find the back window of their car had
: side 25-30 microns of glass are under expansive stress while the inside
: of the glass is under compressive stress.
I'm sorry, but you are wrong here I believe. The surface of the
glass is under compressive stress, as was mentioned in a previous post.
The interior is under tensile stress. Think about it. Tempered glass is
made by rapidly cooling the glass. Now, the surface of the glass will cool
first, which will result in the atoms near the surface "freezing" "solidfying"
however you want to say it, first. Since the interior has cooled less
rapidly, it is still relatively plastic. With continued cooling, the
interior will attempt to contract more than the rigid exterior will allow.
This imposes inward radial stresses such that the surface is put in
compression, whilst the interior is in tension.
The compressive stress that exists at the
surface prevents cracks from propagating as long as they do not get
into the region of tensile stress near the center of the glass. If they
do, then wammo, the glass can almost literally explode. BTW, one
of my professors dealt with several cases involving people being
injured by shattering glass, and it was amazing what he could do with
a bunch of broken glass pieces. I suggest trying to find someone
like my professor, who can "Prove" that the glass could not have failed
due to the impact of the water (might cost you though). Granted, I
am NOT an expert in the failure of glass, but I do know that glass
(especially tempered glass) is very strong, and almost always requires
that a defect be present before it will fail so easily. As for
thermal shock causing the failure, all I can say is it must have been
a hell of a hot day, and the kid was shooting ice water, or he had
his hot chocolate in the gun, and he was in Gnome. Again, even thermal
failure must have been precipitated by the prescence of a defect.
: broken but I almost wrecked the car. What this kid thinks of as a
: harmless prank could have caused the driver to panic and cause major
: damage and (or) injuries.
This is true. But I bet THIS kid won't do it again.
--
___ ______________________
\ \/ __ ______________/ Chris C. Jones jo...@vision.mse.vt.edu
\ / / / "'Bread and Circuses' is the cancer of
\__/ /__/ democracy, the fatal disease for which there is no
cure. Democracy often works beautifully at first.
But once a state extends the voting franchise to
every warm body, be he a producer or a parasite,
that day marks the beginning of the end of the state.
For when the plebs discover that they can vote themselves bread and circuses
without limit and the productive members of society cannot stop them, they
will do so, until the state bleeds to death, or in its weakened condition the
state succumbs to an invader- the barbarians enter Rome."
-R.A. Heinlein
WWW home page at URL http://www.mse.vt.edu/~jones
Dr. Moze
tvo...@unlinfo.unl.edu (Todd Voiles) writes:
> Just a sidebar on this discussion-
>
> The front windshield on cars is NOT tempered (or safety) glass.
>
> It consists of 2 layers of fairly normal glass with a plastic sheet inbetween
>
[edits]
>
> Oh well.....Just my 2 cents as a person with a Ceramic Engineering Degree
>
Good thing they can't issue recalls on science degrees! $^)
----
Dr. Moze (Steve Marsh) ma...@anvil.nrl.navy.mil
Jonathan Priluck
Best regards, Jon Priluck
--
* Jonathan Aerospace Materials Corp., 37 Antwerp St. Brighton MA, 02135 *
* Tel (617) 783-4588, Internet: jam...@world.std.com *
* Developers and future manufacturers of Lattice Block Materials ... *
* the world's strongest and lightest materials. *
Charlie Kuehmann
> If we can show that it is not possible for the
> stream of water from a squirt gun to break the
> window, the prosecutor will drop the charges,
> and our client, who has no record can get
> on with his life.
This probably doesn't answer your question, but my dad used to tell me
when he worked for Libby Glass during the war(WWII) they used to test the
bullet proof glass canopies for fighters not with bullets but with high
pressure water. He said the canopies would deform condiserably and would
be required to withstand a certain pressure before breaking. I don't know
what the canopies were made of or how high the pressures were though.
This is all from rough memory since my dad's been gone for over 10 years
now.
Cheers,
--
If you want the views of my employer ask them. These are mine.
BIRL Industry Research Laboratory
kueh...@nwu.edu
Kelly Miller
>What this kid thinks of as a
>harmless prank could have caused the driver to panic and cause major
>damage and (or) injuries.
If I remember the original post correctly, the kid says that he DID
NOT squirt the car, the driver of the car DID NOT see the kid squirt
the car, nor did she report that the side of her car was wet after
her window broke. She merely ASSUMED that the kid did the damage.
This is despite the fact that she was driving through a construction
site, with gravel that could be kicked up by passing trucks.
Sounds to me like the kid has received a bum rap, even if is theoretically
possible to break a car window by thermal stresses of cold water on a
hot window, or by bb's or some other projectile being shot using a
supersoaker.
Really, what do you think is more likely to break a window -- a flying
rock, or a squirt gun?
Kelly Miller
mil...@aries.scs.uiuc.edu
Allan Duncan
> Hello, I'm working on defending a Juvenille who is
> accused of using a "super-soaker" water gun to
> squirt the car at all - and notes it is in
> a construction zone - that is the scene of the
> incident, so we think it likely that a stone,
> etc. actually did the damage.
Did the driver detect water on the glass?
Was the day hot?
Cold water on hot glass can trigger a latent crack.
So can the smallest shock if the glass is right on the threshold.
Allan Duncan Photonics Section
(+613) 9253 6708 Telstra (formerly Telecom) Research Labs
Fax 9253 6664 Box 249 Rosebank MDC, Clayton, Victoria, 3169
Internet a.du...@trl.oz.au Australia
Quan Li
Does anybody have any idea or clue which company can supply 28/150 EVA resin?
* 28 - VA content; 150 - MFI (Melting Flow Index)
Thank you in advance.
David Allsopp
Flint Smith <fl...@uclaue.mbi.ucla.edu> wrote:
>
>How's this? The "vandal" is wandering through a
>construction site looking for trouble. He finds that
>a pipe cap fits snugly over the end of the supersoaker
>and that it flies nicely when propelled by the stream of
>water. Being a criminal-in-training, it comes naturally
>for him to point his gun at passing motorists. Being only
>interested in guns and destruction, he doesn't know how
>fragile safety-glass is when subjected to point-source
>impacts, or maybe he doesn't care.
Firing solid projectiles with a supersoaker is quite difficult. (I have tried!)
The nozzles of the larger guns are ribbed, so putting anything over the end
doesn't make a good seal, so the pressure isn't very effective, and anything
of noticeable mass will just drop out of the water jet. It won't fly nicely.
David.
-------------------------------------------------------------------------------
It is by caffeine alone I set my mind in motion, it is by the beans of Java
that thoughts acquire speed, the hands acquire shaking, the shaking becomes a
warning, it is by caffeine alone I set my mind in motion.
-------------------------------------------------------------------------------
srb...@sair020.energylan.sandia.gov
>Hello, I'm working on defending a Juvenille who is
>accused of using a "super-soaker" water gun to
>shatter a car window. He states that he didn't
>squirt the car at all - and notes it is in
>a construction zone - that is the scene of the
>incident, so we think it likely that a stone,
>etc. actually did the damage.
>
>If we can show that it is not possible for the
>stream of water from a squirt gun to break the
>window, the prosecutor will drop the charges,
>and our client, who has no record can get
>on with his life.
I suggest that an expert on fractography be consulted. There will
probably be a considerable difference between the type of loading
imposed by a jet of water (probably > 1cm in diameter) and the impact
of a sharp object such as a rock. My guess is that for a super-soaker
to break the glass, there almost had to be a significant flaw on the
window to begin with. The glass pieces might be able to tell the
story. Impact versus thermal cracking can also sometimes be
distinguished. You can get an idea of the use of fractography in
previous court cases by consulting the book: "Failure analysis of
brittle materials", by V. D. Frechette -- published as Advances in
Ceramics, Volume 28, American Ceramic Society, Westerville, Ohio, 136
pp., 1990.
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/==============\
| `63 | IIa | Stephen Brown
|______|_______| Geomechanics Department, MS-0751
/___/^^^^^^\___\9 Sandia National Laboratories
|oo|(@)##(@)|oo| Albuquerque, New Mexico 87185
| | [####] | |
======%%%%====== email: srb...@sandia.gov
{*}={&&}====={*}
{*} {*} RockNet: http://sair019.energylan.sandia.gov:70
-------------------------------------------------------------------------------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dirk Duffner
result...my attorney sued me for unecessary glass wetting with the intent
to fracture. All kidding aside, the side window glass did not fail.
Ken Young
: Hello, I'm working on defending a Juvenille who is
: accused of using a "super-soaker" water gun to
: shatter a car window. He states that he didn't
: squirt the car at all - and notes it is in
: a construction zone - that is the scene of the
: incident, so we think it likely that a stone,
: etc. actually did the damage.
My qualifications are hardware engineering and countless uses of
soakers in "Bloopwar", where grown men with grey hair run around
the parking lot with squirt guns & bloop-shooters.
I know of only one way a soaker _MIGHT_ be able to do this:
Push a headless nail down the barrel, pump it up, and fire.
Such a nail can be found on the ground at a construction site,
so you need to do a little experiment.
Buy an assortment of nails at the hardware store and go to the
auto wreckers. If you can't break a window, your client is clear.
Ken Young
Todd Voiles
Degree
>>
>Good thing they can't issue recalls on science degrees! $^)
>
>----
> Dr. Moze (Steve Marsh) ma...@anvil.nrl.navy.mil
Dr. Moze or Steve March,
I'm just curious about what field(s) your degrees are in? I just
started following this discussion but I do not see the need for simple
childish remarks such as these concerning one's education with no backup.
Just because its on your mind doesn't mean you have to say it.
Gary Krichau
]
] I'm just curious about what field(s) your degrees are in? I just
]started following this discussion but I do not see the need for simple
]childish remarks such as these concerning one's education with no
]backup.
] Just because its on your mind doesn't mean you have to say it.
I need to clarify that I posted the previous post, not Todd Voiles. My error.
Russell Hand
mind:
1) During the initial stages of liquid impact high pressures are
generated. These are known as water hammer pressures and are given (to a
good approximation) by the equation P=rho Cv where rho is the density of
the water (1000 kgm-3), C is the shock wave velocity in the water (to a
frist approximation 1500ms-1, but more accurately given by C=1500+2v,
and v is the impact velocity. These pressures essentially arise from the
geometry of the initial impact and can cause damage if the impact
velocity is high enough. It is possible to measure a threshold velocity
for this damage, which is also effected by the size of the impact drop
or jet of water; for ordinary soda-lime glass this threshold has been
estimated to be 150ms-1 for a 6mm diameter drop and 215ms-1 for a 2mm
diameter drop (data from Hand, Field and Townsend J. Appl. Phys. 70
p7111-7118, (1991)). NB. Such pressures are generated every time that a
car drives through rain thus if a super-soaker could break the
windscreen I suspect that it was in such a state that the next time the
driver drove through rain it would have shattered!!!!
2) Assuming that damage was caused by the super-soaker anywhere near the
threshold velocity it should have only been detectable by residual
strength testing. The high pressures are of very short duration (sub
micro-second usually) and any cracks that are generated do not grow very
much before the stresses are removed and thus the driving force for
growth is removed and thus the cracks stop growing (yes this _is_ the
case with liquid impact on brittle materials - the only catastrophic
failures I ever produced were by firing jets of water at thermally
toughened glass at velocities of 400ms-1 (approx. 800mph) or above - I
find it unlikely that the car/ jet closing speed was anything like this
and if it was I suggest the driver should be prosecuted for exceeding
the speed limit!). As a result the damage from liquid impact is quite
distinctive and produces a ring of short cracks which angle into the
material in a similar fashion to Hertzian cone crack but they _do not_
form a complete cone crack. If a Hertzian cone crack is present at the
fracture origin solid particle impact eg. a stone was responsible. For
liquid impact cracks to grow to the point where catastrophic failure was
possible the windscreen must have been heavily damaged or the car was
travelling at some very excessive speeds.
Basically I think the kid is innocent of the suggested charge and that
a solid particle was probably involved. If a water jet really was the
cause then the driver was being irresponsible driving a car with heavily
damaged windscreen.
Dr. Russell Hand,
University of Sheffield.